One use-case that used to work but no longer does in the Python 3 runqemu rewrite is to copy build artefacts and then invoke bitbake <relevant-image> -c testimage.

Because bitbake is already running the runqemu script can't call `bitbake -e` in order to determine the paths to staging directories.

The old runqemu used which to locate relevant binaries, we could add that as a fallback path in the new runqemu.